This invention concerns a two-beam interferometer for Fourier spectroscopy, particularly for the measuring of radiation in cryostats aboard spacecraft. Its basic components comprise a beam splitter arranged in the path of the source beam, possibly a compensation plate, a first reflector, a second reflector and a detector system for recording the measured radiation.
As is known, two-beam interferometers were traditionally used in the form of Michelson interferometers for exact linear measurements wherein the unknown mechanical lift of the moving reflector is measured with the help of the known, quasimonochromatic measured radiation in the interferometer. The measuring task, which is inverse for this purpose, is the determination of the unknown spectrum of the source radiation fed into the interferometer on the basis of the exactly known movement profile of the moving reflector (Fourier spectroscopy). Fourier spectroscopy as a means for the spectroscopic analysis of unknown radiation was only introduced into metrology at the beginning of the sixties when developments in electronic data processing made available computers for the first time which could numerically evaluate interferograms. For Fourier spectroscopy, the radiation to be investigated is fed into a two-beam interferometer, for example a Michelson interferometer, where it is split into two bundles by means of a semi-penetrable beam splitter. After being reflected back by two mirrors the beams are superimposed and brought to interference. One of the two mirrors is moved in the direction of the radiation by a suitable mechanical drive, whereby the intensity fluctuation of the central spot in the interference pattern is recorded as a function of the position of the movable mirror. The interferogram obtained in this manner is the Fourier transformation of the energy spectrum, i.e. the radiation intensity as a function of the wavelength of the interfering radiation. Fourier spectroscopy is used for the registration of emission spectra as well as absorption spectra. Owing to the improved measuring conditions outside the earth atmosphere, Michelson interferometers have also been carried aboard spacecraft for the measuring of radiation in extraterrestrial areas. From a metrological point of view, this is done in such a way that the detector signals are buffered, if necessary, and are then evaluated in a computer.
Different methods are available for taking interferograms. According to the scanning technique it is sufficient to measure the interferogram only at certain, exactly defined mirror positions. The adjustment and selection of these positions depends on the measurement goal. Besides the classic step-by-step mirror advance, there is also a method wherein the movable mirror is continuously shifted at a constant speed. This method is useful on quickly moving measuring platforms or for measuring short life span radiations.
Typical scanning periods of modern, continuously scanning, interferometers are in the magnitude of one second to about a few minutes per interferogram. In practice, as constant an advance speed as possible is to be obtained with a continuous advance. Speed fluctuations within the range of a few percent are tolerable if vibrations transferred from the drive motor to the moving mirror are suppressed as much as possible. The linear advance of the mirror is normally effected by a spindle or the like riding along one or more guide rails.
Any irregularity in the mirror guide or path, for example a tilting of the mirror plane in the case of a Michelson interferometer, leads to distortions in the interferogram. Owing to the extreme accuracy requirements for the movable mirror guide, high-quality retroreflectors have been used instead of planar mirrors, which retroreflect the incident radiation back in the same direction from which it arrived.
Two-beam interferometers for Fourier spectroscopy which are equipped with movable retroreflectors are known in numerous configurations. Reference is made, for example, to the report of the 1st International Conference on Fourier Spectroscopy in 1970 by G. A. Vanasse et al, published by Airforce Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Mass., U.S.A., pages 43 to 53.
Triple reflectors, cubic corners designed as full prisms, and cat's eyes in reflector or lens designs as well as pentareflectors and pentaprisms have been used as retroreflectors. With a simple replacement of the planar mirrors in a classic Michelson interferometer by such retroreflectors, the linear accuracy of the reflector movement is still required since any lateral traversal of the retroreflector during its movement distorts the interferogram. To counter this problem optical systems have been proposed which are immune to tilting as well as lateral traversal of the reflector. These optical systems are characterized by the combination of a movable retroreflector with a stationary mirror. An example is the "Terrien" system which combines a cubic corner with a stationary reflector (cf., for example, J. Dyson "Interferometry as a Measuring Tool", published by The Machinery Publishing Company 1970, page 92) or the combination of a cat's eye with a stationary reflector (cf., for example, the paper by R. A. Schindler "An Interference Spectrometer for the Remote Sensing of Pollutants" in the Journal Spacecraft, Volume 9, Issue No. 5, page 714).
The present two-beam interferometer in the U.S.A. for measuring atmospheric transmissions, the Atmos experiment for a future spacelab mission, has two cat's eyes as fully compensating retroreflectors with linear guides which are mechanically driven. This drive is considerably complex making the unit relatively heavy and large. Such units may be tolerated for normal measurements in an uncooled environment with a large spacecraft such as a spacelab, but with smaller spacecraft and/or measuring in low temperature ranges in cryostats the most important requirements to be met are low weight and small dimensions as well as extremely low heat generation. High complexity for low temperature measurements in cryostats is necessary for very weak radiations to prevent the inherent radiation of the measuring instrument from blanketing the source radiation. To minimize the refrigerant consumption of the cryostat in long term spacecraft experiments the heat generation in the cryostatic system itself is minimized. Frictional losses in sliding guides, spindle guides and similar mechanical guides of fast moving parts must, therefore, be minimized in the cryoenvironment. This also applies, of course, to the guides of retroreflectors of fast scanning interferometers as required for radiation measurements aboard spacecraft.